This invention relates generally to surface acoustic wave devices and particularly to the wave propagating substrates used therein. Basically, surface acoustic wave devices are comprised of a substrate medium having at least one surface ground and polished to mirror-like finish upon which one or more transducer patterns are formed. In the case of piezoelectric devices to which the present invention relates, the medium is a piezoelectric material and the transducer patterns generally take the form of comb-like metallic structures having interleaved finger pairs. A voltage applied between the "combs" of the launching transducer causes an interelectrode voltage between finger pairs within the transducer which in turn deforms or stresses the piezoelectric medium. In most applications, the voltage applied is a time varying voltage and the successive voltage variations produce varying stresses of the substrate medium causing the propagation of acoustic waves across the surface. In most applications, a second receiving transducer having a structure similar to that of the launching transducer is illuminated by the propagating waves. The mechanical energy of the impinging surface waves is reconverted back to electrical energy in the form of voltage between the comb sections.
In the great majority of surface acoustic wave device applications, the transducer structure is periodic and therefore exhibits a predetermined frequency characteristic in which a band of frequencies (close to the synchronous frequency) causes a maximum interaction with the medium surface while those outside the band cause a minimum reaction. The synchronous frequency for any given combination of substrate material and transducer arrangement is determined largely by the relationship between spacing of the transducer comb elements and surface wave velocity of the piezoelectric medium. As a result, consistency of frequency response of mass produced surface acoustic wave devices requires that the surface wave velocity of successive substrates remain within a narrow range of acceptable tolerance. In addition, the most commonly used method of forming transducer structures on the substrate surface involves initially depositing a uniform metallic film across the entire surface and photo-etching to remove undesired metal and produce a transducer pattern. As a result, a substantially uniform fine-grain structure in the material is needed for proper transducer formation.
Piezoelectric surface acoustic wave devices have been commercially manufactured using either lithium niobate or lead zirconate titanate (generally abbreviated PZT) materials. The former is a single-crystal medium so-called because it is fabricated from a single large elongated crystal formed by conventional crystal growth processes. The elongated crystal is "poled" by application of an appropriate DC electric field to impart a piezoelectric characteristic to the material. The poled elongated crystal is then crystallographically aligned and sliced into a number of substantially planar disks. The disks are then ground and polished by successively finer abrasives until a mirror-like surface is obtained upon which a plurality of identical transducer patterns (typically 40 or so) are photo-etched. Finally, the disks are scored between the individual transducer patterns permitting separation of individual surface acoustic wave devices.
PZT is a polycrystalline material so-described because the constituent components of lead, zirconium and titanium in the form of metal oxide powders are essentially ground to a fine powder which is blended to form a homogeneous mixture. A "hot pressing" technique in which high temperatures and pressures are simultaneously imposed upon the mixture causes the materials to react and form an elongated "boule" of PZT. The boule is then processed in a manner similar to the elongated crystal of lithium niobate, that is, the boule is poled to impose piezoelectric properties, sliced to form a plurality of planar disks, and subjected to successive grinding and polishing operations to produce a smooth surface upon which a plurality of transducer structures are formed.
Acceptable commercial results of device consistency have been achieved with both single crystal lithium niobate and polycrystalline PZT. However, the fabrication costs associated with such substrates has resulted in generally prohibitive surface acoustic wave device costs making their infusion into consumer electronic applications slow despite their substantial performance advantages.
Another polycrystalline material, lead titanate exhibits considerable promise as an acoustic surface wave medium. It is in some sense similar to PZT but enjoys economic advantages over both lithium niobate and PZT. In addition, lead titanate has a more favorable ratio of thickness coupling to radial coupling than PZT which simply stated, means that a greater portion of the electrical energy imparted to the transducer is converted into surface acoustic waves rather than undesired bulk mode waves. Also the Curie temperature of lead titanate is higher than PZT which means essentially that the piezoelectric properties, once imparted to lead titanate, remain unaffected when the substrate is subjected to higher temperature processing and operating environments. And finally, lead titanate has a low dielectric constant and a high mechanical "Q" making it well suited to high frequency signal operations.
Conventional titanium oxide processes and ceramic "air fired" methods have been used to manufacture lead titanate suitable for use in bulk mode piezoelectric resonators. Examples of these "oxide" processes are found in U.S. Pat. No. 3,642,637 entitled "Piezoelectric Ceramic Composition" issued Feb. 15, 1972 and in scientific publications such as "Effects of Additives in Piezoelectric and Related Properties of PbTiO.sub.3 Ceramics", Japanese Journal of Applied Physics, Vol. 11, No. 4, April 1972, p. 450, and "Electromechanical Properties of PbTiO.sub.3 Ceramics", The Journal of the Acoustical Society of America, Vol. 50, No. 4 (Part 1), p. 1060.
Application of lead titanate as a medium for surface acoustic wave devices, however, has thus far been subject to a substantial disadvantage which has limited its commercial success. This limitation arises due to the difficulties associated with fabricating lead titanate substrates which are mechanically stable and have the sufficiently uniform fine-grain structure required to produce the uniform surface wave velocities so essential to mass production. Some success has been realized by the utilization of hot pressing techniques similar to those associated with PZT together with lanthanum dopants, however, such hot press fabrication is subject to the same economic disadvantages associated with formation of PZT.
Accordingly, it is an object of the present invention to provide an improved method of fabricating a lead titanate surface acoustic wave substrate. It is a further object of the present invention to provide an economical cost-effective method for producing fine grain lead titanate surface acoustic wave substrates having acceptable consistency of properties.